Web browsers have to use the Safari engine but that doesn't mean they don't also separately update the browsers and add functionality. I.e. you mostly get modularity by not using Safari even if you have to rely on the underlying Safari engine being updated. Most of the features users notice are updated by the app provider anyway.

Does the browsee limitation come from loading non reviewed code and interpreting it? Are other browsers allowed if they don't implement JavaScript?

No I'm not being sarcastic - I think calling the apps a skin on top of Safari is a bit of an over-simplification. Most features that users notice happen at this skin layer, not the rendering layer. It's peak HN to really be caring about this especially as Apple offers (as far as I know) no performance difference across browsers using Webkit or w/e. You could actually just say that Safari itself is a skin too, just the default one that comes with iOS.

It's similar to complaining about other basic features of iOS IMO (like complaining there's a default settings app or that iOS just works a certain way).